This invention generally concerns a method and apparatus for dynamically sizing and operating enable groups of thermal elements in a printer to allow it to operate with a variety of power sources having different outputs.
Thermal printers include a single row of thermal elements for printing a single line of an image onto a print medium such as paper. Each thermal element is in reality an electrical resistance heater that fuses dye from a donor onto the print medium. As such, each element draws a significant amount of electrical current whenever it is actuated. As the number of thermal elements in the printhead of such a printer may number between 512 to several thousand, a large capacity power source is necessary if all of the thermal elements are to be driven simultaneously. To obviate the need for such a large capacity power source, it is known in the prior art to wire the thermal elements so that they are divided into two or more enable groups which may be operated at different times. The use of such enable groups allows the printhead to be driven by a power source whose maximum output is insufficient to actuate all of the thermal heating elements at once. In operation, current from the power source is sequentially multiplexed to the thermal elements in each of the enable groups, each group of which is small enough to be actuated by the power source without overtaxing it. The division of the thermal elements into such enable groups advantageously allows the printer to be powered by a smaller and less expensive power source than would otherwise be necessary at the expense of a longer printing time.
Unfortunately, there are a number of shortcomings associated with the hard wiring of the thermal elements into a fixed number of enable groups. However, before these shortcomings can be fully appreciated, some background as to the overall structure and operation of such thermal printers is necessary.
The thermal elements in such printers are intermittently connected to the power source via switches in the form of NAND gates. The NAND gates either connect or disconnect their respective thermal elements to the power source in response to "0" or "1" data bits received from a latch circuit. Each of the latch circuits is in turn connected to a one-bit wide gate of a shift register which receives a stream of image data from a microprocessor. In operation, the shift register serially loads data bits from the stream of image data into the latch circuits through its gates in accordance with clock pulses supplied by the microprocessor. In a six-bit printer, 64 possible data in the form of "1s" and "0s" are admitted through each shift register gate for every line printed by the thermal printhead, which in turn allows the printhead to generate 64 different shades of a color per pixel.
In a printhead having 512 resistive printing elements, each of the elements requires approximately 48 milliamps at 24 volts every time it is actuated by a "1" signal relayed to its respective NAND gate via a latch circuit. If all of the thermal elements were actuated simultaneously, the load on the power source would be 24.6 amps. As many power sources are not capable of delivering such a current, the thermal elements in some prior art printheads were divided into enable groups which were sequentially operated in order to reduce the load on the power source. For example, if it was desired to utilize a power source having a 6.5 ampere capacity at 24 volts, the 512 thermal elements of the printhead would be hard-wired into four enable groups of approximately 128 elements each. In operation, current from the power source would be serially multiplexed to each of the four enable groups to complete a line of printing before the paper and the dye donor were moved relative to the printhead in anticipation of the printing of the next line of the image.
While the hard-wiring of the switching circuits into a specific number of enable groups allows a printhead to operate off of a smaller and less expensive power source, it also complicates both the structure and operation of the printhead. For the smaller-capacity power source to sequentially activate the thermal elements in a particular enable group, the base lead of each of the NAND gates must be connected to different wires to allow the multiplexing of electrical power. The microprocessor needs to generate not only the NAND-gate controlling image data stream, but also the multiplexing commands necessary to sequentially operate the enable groups. Most importantly, such hard-wiring creates a fixed number of enable groups that limits the use of the printhead to a particular power source. Such prior art printheads cannot be used at all with a power source that does not have the capability of operating at least one of the enable groups. This is particularly problematical in prior art printheads that have few or only one enable group. Conversely, if such a prior art printhead is used in conjunction with a higher capacity power source, it may not be possible to increase the speed of printing even by modifying the multiplexing signals generated by the microprocessor.
Clearly, there is a need for a thermal printhead which is not only capable of being used by power sources having substantially different outputs, but which also makes optimal use of the power received in terms of print speed when connected to a higher output power source.